John Allen, former children's home boss, had no child care qualifications when he set up his company. Photograph: National Crime Agency/PA

The owner of a group of children’s homes has been jailed for life after he was found guilty of sexually abusing 19 vulnerable children who were in his care over more than 20 years.

John Allen, 73, who was the owner and manager of the Bryn Alyn Community homes in north Wales, abused children as young as seven from the late 1960s to the early 90s.

Telling Allen that he would spend at least 11 years behind bars before being considered for parole, Mr Justice Openshaw said he had used his position of power to abuse children from “dysfunctional [and] chaotic” backgrounds who desperately needed attention and love.

The judge said: “Seldom can there be so many grave offences committed on so many victims over a long period of time, causing so much anguish and misery.”

Openshaw said it was “remarkable” that in those “far-off days” there were no regulations requiring Allen, a hotelier by trade, to have formal training before he set himself up as the boss of a string of care homes in and around the Wrexham area of north Wales.

The judge said supervision and monitoring of Bryn Alyn was “wholly inadequate”. He said: “The boys did not complain because they knew it would pointless to do so and it gave the belief that the defendant was untouchable.”

Openshaw said that some of the complainants, 18 boys and one girl, had led “sad and troubled lives” following their experiences at the Bryn Alyn homes. “Some have had nervous breakdowns. A number have attempted suicide. Some have been in prison,” he said.

One victim said Allen “had been with him for every second of his life”, another said Bryn Alyn had “fucked my life up” after he turned to drink and became homeless. The judge reminded the court that one victim told the jury “with infinite sadness” that “it was as if they had taken my manhood”.

Several of the victims were in court to see Allen sentenced and there were gasps in court when his barrister, John McDermott QC, said the defendant maintained that he was innocent of all 33 charges he was found guilty of. Allen showed no emotion as he was led from the dock.

The conviction is the first since detectives launched an inquiry called Operation Pallial into historical child abuse in and around Wrexham. Led by the National Crime Agency (NCA), Pallial was started in the wake of the discredited BBC Newsnight report in which a former children’s home resident claimed he had been abused by a senior Conservative party figure from the Thatcher era.

Lord McAlpine was widely identified on Twitter and elsewhere on the internet as the Tory referred to, until a Guardian investigation concluded that he was the victim of mistaken identity. By then dozens of victims had come forward to say that they had suffered abuse and to claim their complaints were not taken seriously at the time, sparking the NCA investigation.

So far, allegations made by 236 people are under “active investigation” and 120 people have been named as potential suspects. Thirty-one men and four women have been arrested or interviewed under caution and of those 12 are due to stand trial for various offences in 2015.

Allen was working as a night porter at a hotel in Suffolk when the Newsnight report ran. He has already been in prison for abusing children after being found guilty in the early 1990s of sexual abuse relating to six boys. In 2001 he was charged with sexual offences connected to a number of boys but was not put on trial because of a technicality.

During his trial at Mold crown court in north Wales the prosecution described how he created a “sexualised atmosphere”, grooming some children by giving them gifts, including motorcycles, and treats such as lunches out but threatening others with violence if they did not comply. Children were assaulted in their dormitory beds, in bathrooms, during camping trips and night-time expeditions, at Allen’s home and at his hotel.

Those who plucked up the courage to go to the authorities were disbelieved or ignored, it is claimed. The court was told that one victim alleged he saw Allen assault a child in front of a social worker without any comeback.

Another boy claimed he told a social worker he had been abused only to be assaulted by the social worker’s boss.

When youngsters did go to the police, Allen was called to pick them up and take them back. One boy alleged that he was introduced to Allen through a “paedophile gang” before being abused by him.

A separate inquiry chaired by Lady Justice Macur is investigating an earlier investigation by the late Sir Ronald Waterhouse into the abuse of children in care in the former Gwynedd and Clwyd council areas of north Wales between 1974 and 1996. Many former children’s homes victims have criticised the scope and findings of the Waterhouse review.

Commenting on the NCA investigation, Keith Towler, children’s commissioner for Wales, said: “When allegations of historical sexual abuse in the north Wales care system made headline news in 2012, I was insistent that those victims must be listened to, supported and dealt with sensitively. For too long, these victims were silenced.

“Two years on, I am reassured that victims have been heard and that instances of historical abuse are finally being thoroughly investigated by Operation Pallial.”

Jon Brown, NSPCC lead for tackling sexual abuse, said: “John Allen’s reign of terror over a dark period of several decades puts him high up the scale of the most prolific child abusers of recent times and his sentence quite rightly reflects the severity of his despicable crimes.

“Instead of nurturing the vulnerable children in his care he abused his power and subjected them to constant, horrendous sexual attacks. His manipulative and devious behaviour allowed him to hide behind a shroud of respectability while his helpless victims suffered intense emotional trauma. Allen was able to evade justice for so long because he created an atmosphere of terror where no one was able to oppose him and where he effectively silenced his victims.